1. Field of the Invention
The present invention relates to the production of cerium oxides, zirconium oxides, cerium and zirconium mixed oxides or solid solutions (also hydroxides and carbonates) having improved thermal stability and oxygen storage capacity. The oxides, hydroxides or carbonates have a fine particle size distribution, very high surface area, oxygen storage capacity and release capacity, and are useful in many applications including catalytic converters, catalysis for the manufacture of styrene and catalysis for gas exhaust systems.
2. Background of the Invention
Oxides of cerium and zirconium, and particularly cerium and zirconium (Ce,Zr)O.sub.2 mixed oxides and solid solutions, are used for many applications, including catalysts used in automotive catalytic converters and the like. Such oxides are typically formed by known precipitation techniques which involve the formation of precursors to, or the solid oxides in a liquid medium. When such oxides are to be used, for example, in catalytic converters, it is desirable to maximize the thermal stability of the compounds, as defined by the stability of the surface area of the material after aging at high temperature. It is also desirable to maximize the surface area of such mixed oxides in order to provide improved catalytic properties. In addition to (Ce,Zr)O.sub.2 mixed oxides, the present invention also relates to cerium oxides, zirconium oxides, and mixtures thereof as well as cerium and zirconium (Ce,Zr)O.sub.2 solid solutions (where substitution between cerium and zirconium in the network of the oxide, as opposed to being two different phases, which can be utilized as catalysts or as catalyst supports.
Increasingly stringent vehicle emissions standards make exhaust system operating conditions increasingly severe. The majority of modern gasoline fueled cars are equipped with so-called three-way catalysts to aftertreat their exhaust gases. The purpose of this system is to convert simultaneously carbon monoxide, hydrocarbons and nitrogen oxides by means of a precious metal based heterogeneous catalyst, whereby the engine's air-to-fuel ratio is controlled to obtain exhaust gas compositions that guarantee optimal conversions. Cerium-zirconium oxides are widely used in three-way catalysts for automotive exhaust treatment. Three-way automotive catalysts consist of precious metals (platinum, rhodium, etc.), promoters and supports such as .gamma.-alumina. It is known that the addition of CeO.sub.2 as a promoter results in an improvement of the dynamic performance for the removal of carbon monoxide (CO), nitrogen oxides (NO.sub.x) and hydrocarbons (Hcs). However, the high temperature conditions of use in an automotive engine lead to significant degradation including surface area loss of supports, sintering of supported precious metals and deactivation of added cerium.
It is known that cerium oxides and zirconium oxides and cerium and zirconium mixed oxides can be used as a catalyst or as a catalyst support. It is also well known that a catalyst is generally more effective when the contact surface between the catalyst and the reagents is larger. For this purpose, it is necessary for the catalyst to be maintained in the most divided state possible, that is, that the solid particles which compose it be as small and individualized as possible. The fundamental role of the support, therefore, is to maintain the catalyst particles or crystallites in contact with the reagents, in the most divided state possible. During the extended use of a catalyst support, a decrease in the specific surface occurs due to the coalescence of the very fine micropores. During this coalescence, part of the catalyst is surrounded by the body of the support and can no longer be in contact with the reagents.
An object of the present invention is to provide a method for producing cerium oxides, zirconium oxides, cerium and zirconium (Ce,Zr)O.sub.2 mixed oxides, and cerium and zirconium (Ce,Zr)O.sub.2 solid solutions having improved thermal stability, surface area, porosity, and/or oxygen storage capacity. The method is preferred for use producing cerium oxides, (Ce,Zr)O.sub.2 mixed oxides and (Ce,Zr)O.sub.2 solid solutions, having improved thermal stability, surface area, porosity, and/or oxygen storage capacity.
Another object of the present invention is cerium oxides, zirconium oxides, (Ce,Zr)O.sub.2 mixed oxides, and (Ce,Zr)O.sub.2 solid solutions compositions having improved thermal stability surface area, porosity, and/or oxygen storage capacity. The oxides, mixed oxides and solid solutions produced can have very high surface areas, very high oxygen storage capacities and low particle size.
These and other objects of the present invention will be more readily apparent from the following description.